Metric and imperial conversions: why a gallon is not always a gallon
A conversion is one multiplication once you know the exact size of each unit. The trouble is that one name can cover several sizes. A US gallon is 3.785411784 litres, a UK gallon is 4.54609 litres, and the cup on a nutrition label is not the same as a US cup. Convert through a single base unit, keep full precision until the last step, and the result is exact for the definitions you chose.
Unit converterWhat is that in metric?OpenConvert through one base unit
Every unit has a fixed size in a base unit: the metre for length, the kilogram for weight, the litre for volume. To convert from A to B, multiply by A's size and divide by B's size. Ten miles is 10 × 1,609.344 metres, which is 16,093.44 metres, or 16.09344 km. You only ever need one size per unit, not a separate formula for every pair.
That is also why the unit converter can show a value in every unit of a category at once. It converts into the base unit once, then into each unit in turn.
Length and weight are exact
The imperial units are defined in metric terms, so these conversions are exact:
| Unit | Exact size |
|---|---|
| 1 inch | 0.0254 m |
| 1 foot | 0.3048 m |
| 1 yard | 0.9144 m |
| 1 mile | 1,609.344 m |
| 1 nautical mile | 1,852 m |
| 1 pound | 0.45359237 kg |
| 1 ounce | one sixteenth of a pound, about 28.35 g |
| 1 stone | 14 pounds, about 6.35 kg |
A stone is a British unit for body weight, and it is 14 pounds, not 10. The BMI calculator takes stones and pounds directly, so there is nothing to convert first.
Temperature needs formulas
Temperature scales start from different zeros, so a single multiplier cannot convert them. The formulas are °F = °C × 9/5 + 32 and K = °C + 273.15. So 100 °F is (100 − 32) × 5/9, which is 37.78 °C. Zero degrees Celsius is 32 °F and 273.15 K. The two scales cross at −40, where −40 °C and −40 °F are the same temperature.
Volume: where most mistakes happen
- A US gallon is 3.785411784 litres. A UK (imperial) gallon is 4.54609 litres, about 20% bigger.
- A UK fluid ounce is smaller than a US one: 28.41 ml against 29.57 ml. The UK gallon is divided into 160 fluid ounces, not 128.
- A US customary cup is 236.588 ml, which is 8 US fluid ounces. US nutrition labels use a 240 ml cup, about 1.4% larger.
- A US tablespoon is 14.79 ml and a US teaspoon is 4.93 ml.
- A US liquid pint is about 473 ml and an imperial pint is about 568 ml.
If a recipe comes from one country and a label from another, check which gallon, pint or cup is meant before you scale anything.
Area and speed
An acre is 4,046.8564224 m², so it is a little under half a hectare, which is 10,000 m². A mile per hour is 0.44704 m/s. A kilometre per hour is 1/3.6 m/s. A knot is one nautical mile per hour, which is 1.852 km/h.
Bytes: KB is not KiB
Storage uses two systems. Decimal units count in thousands: 1 KB is 1,000 bytes, 1 MB is 1,000,000 bytes and 1 GB is 1,000,000,000 bytes. Binary units count in 1,024s: 1 KiB is 1,024 bytes, 1 MiB is 1,048,576 bytes and 1 GiB is 1,073,741,824 bytes. The gap is 2.4% at the kilobyte level and about 7.4% at the gigabyte level.
Drive makers use decimal units. Many operating systems report binary sizes under the decimal label, so a 1 TB drive can show as about 931 GB. The drive has not shrunk. The two systems are counting the same bytes in different units.
Keep full precision
Round only at the end. Rounding an ounce to 28 g and multiplying by 16 gives 448 g for a pound, against the exact 453.59 g. Keep six or more significant figures while you calculate, and round the answer to the precision the label or recipe needs.
Edge cases
- Ton. A short (US) ton is 2,000 pounds, a long (UK) ton is 2,240 pounds, and a metric tonne is 1,000 kilograms.
- Dry and liquid pints. A US dry pint is a different size from a US liquid pint.
- Square units. A square foot is 0.09290304 m², which is 0.3048 squared. Converting square units with a length factor gives the wrong answer, so square the factor.
Quick answers
How many millilitres are in a US cup? 236.588 ml, which is 8 US fluid ounces. Nutrition labels use 240 ml.
Is a stone a unit of weight? Yes. A stone is 14 pounds, about 6.35 kg.
Why does a 1 TB drive show as 931 GB? The drive is sold in decimal units, while the operating system reports binary units under a decimal label.
For fuel economy in litres per 100 km, the fuel cost guide uses the same gallon and mile conversions.
